1. Field of the Invention
The present invention relates to binder resins for sliding resistors included in potentiometers for use in, for example, on-vehicle sensors, and also relates to sliding resistors formed using the binder resins. In particular, the present invention relates to a binder resin that can provide a sliding resistor capable of withstanding 50 million, or more than 100 million, sliding movements, and a sliding resistor formed using the binder resin.
2. Description of the Related Art
A potentiometer is generally used as a variable resistor in, for example, on-vehicle sensors. This device includes a resistor that is typically formed with a binder resin in which conductive particles, such as carbon black, are dispersed, and a slider, such as a metal contact brush, that can be moved over the resistor to change its resistance.
The binder resin used is generally a thermosetting resin such as phenol resin and epoxy resin. Examples of the conductive particles used include carbon black and carbon fiber, which also serves to increase the durability to sliding movements. The resistor is formed by mixing and dispersing the conductive particles into the binder resin together with a solvent to prepare a resistor paste, applying the resistor paste directly onto an insulating substrate by, for example, screen printing, and firing the resistor paste.
For example, sliding components such as on-vehicle sensors installed in the vicinity of engines are exposed to increased temperatures of about 100° C. to 120° C. Sliding components used in such a hostile environment therefore need a resistor having high heat resistance. In recent years, polyimide resin has been frequently used as a binder resin for applications such as on-vehicle devices because this resin has high heat resistance.
Japanese Patent No. 3372636 (the corresponding U.S. Pat. No. 5,781,100), for example, discloses a method of producing a resistor substrate having excellent heat resistance, less variations in resistance due to heat, and a long life using an acetylene-terminated polyisoimide oligomer having a specific glass transition temperature as a binder resin.
A resistor substrate produced by the above method, however, has the problem that more than 50 million sliding movements cause an increase in contact resistance due to the wearing down of the resistor. On-vehicle sensors, for example, need a potentiometer having the durability to 50 million, or more than 100 million, sliding movements. Resistors included in potentiometers are therefore required to achieve a further improvement in sliding durability.